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    Subject[PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit
    When a 64-bit x86 processor runs in 32-bit PAE mode, a pte can
    potentially have the same number of physical address bits as the
    64-bit host ("Enhanced Legacy PAE Paging").

    This is a bugfix for two cases:
    1. running a 32-bit PAE kernel on a machine with
    more than 64GB RAM.
    2. running a 32-bit PAE Xen guest on a host machine with
    more than 64GB RAM

    In both cases, a pte could need to have more than 36 bits of physical,
    and masking it to 36-bits will cause fairly severe havoc.

    The 46-bit mask used in 64-bit seems pretty arbitrary. The physical
    size could be between 40 and 52 bits. Setting the mask to 40 bits
    would restrict the physical size to 1TB, which is definitely too
    small. Setting it to 52 would be ridiculously large, and runs the
    risk that one of the vendors may decide to put flags rather than
    physical address in one of the upper reserved bits.

    Doing it "properly" would require testing cpuid leaf 0x80000008, but
    it would mean that we would lose the ability to make all these
    compile-time constants.

    So, stick with 46 bits. It's enough for now.

    [ Ingo: This needs a test, but I think it should be fairly low-risk.
    If it checks out OK, it should be slipped to Linus fairly soon,
    since it is a bugfix. It's probably worth putting into stable
    too. ]

    Signed-off-by: Jeremy Fitzhardinge <>
    Cc: Jan Beulich <>
    Cc: Stable Kernel <>

    diff -r 0eebd30011dc include/asm-x86/page_32.h
    --- a/include/asm-x86/page_32.h Wed Jun 04 10:32:01 2008 +0100
    +++ b/include/asm-x86/page_32.h Thu Jun 05 16:09:53 2008 +0100
    @@ -22,7 +22,7 @@

    #ifdef CONFIG_X86_PAE
    -#define __PHYSICAL_MASK_SHIFT 36
    +#define __PHYSICAL_MASK_SHIFT 46
    #define __VIRTUAL_MASK_SHIFT 32
    #define PAGETABLE_LEVELS 3

     \ /
      Last update: 2008-06-05 17:25    [W:0.027 / U:0.052 seconds]
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